In one type of demagnetizing system of the prior art, a part to be demagnetized is placed within a coil which is connected to an AC line and the part is then withdrawn from the coil until it is well away therefrom after which the energization of the coil may be discontinued. The purpose is to apply magnetic flux in reversing directions starting out with a high flux initially and then gradually reducing the flux down to a very low value.
In another type of system, a part to be demagnetized is placed within a coil to which a current of alternating polarity is applied in a programmed manner, with the current being gradually reduced until it is at a very low value.
The latter type of system, in which a programmed decreasing current is applied, is not suitable for elongated parts because of difficulties in obtaining a coil large enough to apply a field to the entire part. The first type of system, in which the coil is connected to an AC line, is suitable for demagnetization of elongated parts in that such parts can be moved longitudinally through the coil to cause a magnetic field of alternating polarity and of reducing magnitude to be applied to each portion of the part as it moves through and beyond the coil. However, if such parts are inspected and tested afterward it will be found that in many cases, the parts are nowhere near being completely demagnetized and as a result, problems frequently occur. As an example, drill pipe which has become strongly magnetized during a magnetic inspection for defects may be passed through a coil connected to an AC line for the purpose of demagnetizing such pipe but when welding operations are subsequently attempted on the pipe, difficulties may be encountered which are due to strong residual fields within the material of the pipe. With other types of parts, other problems may occur such as the attraction of cuttings and chips to the surfaces of parts on which machining operations are performed.